I have had intermittent issues with copy/paste from Audacity labels to other labels and have solved this by pasting and copying in/from text editor. But now no sort of paste works for track labels. Copying from label text to text editor also does not work.
I reverted to 2.4.0 and it works.
This is fixed in version 3.0 alpha. Release date is not known for sure, but likely some time in February.
– Bill